Doped zirconia ceramics are known and can have various configurations of crystal structure. For example, zirconia ceramics can have a cubic phase crystal grain structure. A disadvantage of ceramics having the cubic phase is that they are brittle and therefore easily break from impact or thermal shock. Another form of zirconia ceramic has both the cubic phase and a monoclinic phase, and exhibits improved strength and thermal shock resistance compared to cubic phase ceramics. Both such types of zirconia ceramics, however, tend to rapidly develop cracks which can deteriorate material strength and integrity, thus limiting their usefulness for applications such as cutting tools. It is also necessary to machine both types of ceramics after sintering to obtain a tool having a desired net shape that can be employed for its intended use, adding to the fabrication cost.
Ceramics having a tetragonal phase crystal structure have also been described, generally with the cubic and/or monoclinic phases present in varying proportions. Such ceramics can have improved strength and improved resistance against deterioration in strength compared to the above-described ceramics that do not have a tetragonal phase. These ceramics, however, can also require a post-sintering machining operation, unlike a zirconia ceramic consisting essentially of the tetragonal phase. U.S. Pat. No. 4,360,598 describes tetragonal phase zirconia ceramics and their improved dimensional stability, and discloses a zirconia ceramic having a tetragonal phase that is substantially cubic and monoclinic phase-free. The ceramic, however, consists essentially of the tetragonal phase at just one discrete value of dopant loading, limiting the usefulness of the ceramic and the process.
It is therefore desirable to provide a zirconia ceramic, and a process for producing same, that consists essentially of the tetragonal crystal phase over a wider range of dopant loading than is presently known, and which ceramic can be formed to a net shape.